[2022-08-09]: Head of so-called Donetsk People's Republic (and former Ponzi scheme CEO) Denis Pushilin declared on live tv (Соловьёв Live) that he expects the arrival of the first brigades of North Korean workers soon. He also said that DPR and North Korea have talks about collaboration when it comes to building [1].
It is probably the most direct admission you can get right now. Why? Because there are UN Security Council sanctions against North Korea prohibiting the procurement of services from North Korean companies, including building services. Also, just a week ago, Russian denied violating the sanctions related to North Korea, stating something along the lines of "Russia being a responsible member of international society" [2].
As far as I know from my personal experience, when Russia makes claims denying involvement in something, it is usually involved in that thing.
There are also statements about North Korean workers being sent to Ukraine on South Korean website about North Korea [3]. Just "we have information about that", but it looks plausible and completes the picture.
[1] https://tass.ru/mezhdunarodnaya-panorama/15427835
[2] https://ria.ru/20231015/kndr-1902858034.html
[3] https://www.dailynk.com/english/north-korea-orders-trading-c...